The Study On Environmental And Economical Benefits Of Integrated Urban Sewage Treatment Systems Based On Emergy

Wastewater treatment is one of the most significant methods to address the water pollution and water resource exhaustion related issues in China. Under the background of the fast economic growth and the ecological civilization building, wastewater output is unceasingly growing while the related discharge standards are more stringent. Therefore, more sewage treatment plants have been constructing to meet the challenges.Wastewater treatment is a method which cannot remove all the pollutants completely incompletely. Meanwhile, some secondary pollutants, such as air pollutants, excess sludge may be produced during the course of sewage treatment plant construction and operation. Hence, the system boundary of wastewater treatment should be extended as well, i.e. wastewater treatment, treated water treatment and sludge treatment are integrated into one whole system. Only by this way can the economical environmental benefits be evaluated systematically and comprehensively.Due to assessing environment, economy and society based on one common measure, emergy analysis has been widely used to evaluate all kinds of systems with different scales. This method considers natural contribution to economic activities, and it concentrates on the interaction between human and nature and the related resources efficiency and environmental impact. Therefore, it is one of helpful tools to evaluate environmental-economic performances of these complex systems controlled by human and environment, e.g. an integrated sewage treatment system.In the frame of emergy theory, this research has built five wastewater treatment scenarios regarding the Dayi County sewage treatment plant in Sichuan Province as a case, i.e. scenario 1:discharging wastewater directly; scenario 2:wastewater treatment+treated water treatment+sludge landfill; scenario 3:wastewater treatment+treated water treatment+sludge vermicomposting; scenario 4:wastewater treatment+treated water reuse+sludge landfill; scenario 5:wastewater treatment+treated water reuse+sludge vermicomposting. Next this research has put forward the following indicator system to assess the comprehensive performance of the two five scenarios, composing of net income of emergy (NIE), ecological net income ratio of emergy (ENIRE), environmental load ratio (ELR), sustainability index (SI) and a vicarious value accounting method for waste recycling. The study result shows that:(1) According to the result of emergy flows, as far as directly input is concerned, scenario 1 needs no input; however, scenarios 2-5 highly depend on external resources. Scenario 3 and 5 had reasonable resource structure (higher renewable input ratio) than scenario 2 and 4. As for the required environmental services, it slowly decreases from scenario 1 to 5 with the value of 3.88×1019sej,8.45×1018sej,8.44×1018sej,4.89×1018sej and 4.88×1018sej, respectively. This result shows that wastewater treatment, sludge treatment and sludge vermicomposting can effectively reduce environmental services. As for the final economic output, it rises steadily from scenario 1 to 5, with the value of 0 sej, 0 sej,1.64×1016 sej,5.37×1018 sej and 5.39×1018 sej, respectively. It is found that wastewater reuse and sludge vermicomposting can increase revenue. Generally wastewater treatment and treated water reuse can improve environmental and economical benefits simultaneously.(2) According to the indicator results, scenarios 4 and 5 have higher emergy net benefits than plan 2 and 3. Therein, scenarios 2 and 5 have positive environment net benefit, scenarios 4-5 have positive product net benefit, and scenarios 2-3 have negative product net benefit. Wastewater treatment and treated water reuse have higher economic benefits than sludge vermicomposting. As for ecological emergy net benefit ratio, scenario 4 ranks the first, and then followed by scenario 5,2 and plan 3 accordingly. The environmental load ratios of scenarios 2-5 is reduced orderly; on the contrary, the sustainability index for the corresponding scenarios is increasing in sequence. It is found that "wastewater treatment+treated water reuse+sludge vermicomposting" was the best one, which optimized both resources efficiency and environmental load. Meanwhile, the trends of indicator ELR was completely opposite to that of the indicator SI for scenario 2-5, which means that ELR was the main factors influencing the sustainability of sewage treatment integrated systems. Sludge vermicomposting and treated water reuse can reduce environmental load through different ways:the former improved the comprehensive performance of sewage treatment process through optimizing input structure and increasing renewable resources ratio (especially external renewable resource ratio), while the later achieved the related propose through reducing environmental services required.(3) Comparing to the existing indicator system, the proposed indicator system take economy and environment into consideration, which was more precise to characterize treatment processing system.(4) The value of reclaimed water cannot be considered to be that of fresh water or tap water. According to this research, the value of the reclaimed water is 3.54 times of that fresh water, or 1.34 times of that of tap water. The accounting method of substituted value based on emergy theory is able to assess the value derived from pollution reduction and resources-saving in the process of reclamation of wastes.